Obviously from a strategic standpoint you're going to need a raid group to get far in a contemporary raid instance. Just zoning in though, that also requires a 'raid'. There is this strange requirement that players be in a raid group in order to zone in. It's an inconsistent requirement.

The raid group does not even need to be a group. Five players in a group cannot zone in, but hit the convert to raid button and now they can, despite no actual difference in the group composition. This was a past-time in the past, of completing a BRD run and attuning people and then you'd convert to a raid and gaze in wonder at the Molten Giants that stand near the entrance. Sometimes we'd pull one, they're linked so we'd get both, and see what happens. Usually it meant we died. Sometimes we ran out fast enough.

You don't need 40 people to zone in to a 40-man raid. You don't need 25 for a 25 or 10 for a 10. Or even two. The other person in the 'raid group' can be offline, even on another character, and it still works. I don't know why the raid group requirement exists. Maybe it was a way to prevent the generation of hundreds of single-person raids that would destroy the servers back when Blizzard staff wasn't yet swimming in pools of molten gold. BTW, a very painful process, but they come out very shiny.

The inconsistency goes further. It seems to me that the raid group check only occurs at instance portals. That means that if you can get inside without the glowing purple or green wall curtain, you don't need a raid. There are two raids which have that ability. It's not coincidence that both are also linked to instances. Molten Core is linked to Blackrock Depths and has a portal down there, which no one uses (anymore), but also in Blackrock Mountain there is a window which acts like a portal. The window does the raid check. But next to it, there is an elf. He doesn't. Talk to him and you can get in, even without being in a group, let alone a raid. Blackwing Lair is linked to Blackrock Spire, and also has a teleport mechanic in Blackrock Mountain, an orb which will put you inside the raid instance, again, without needing to be in a raid group.

I've tried other raid instances, not all, but a decent sample covering both sides of Ahn'Qiraj, Karazhan, Mount Hyjal in the Caverns of Time, Icecrown Citadel, and Ulduar. All block me, requiring a raid group. This seems to support my theory that if you can bypass the portal, you can get inside. But I don't know of any other raids which allow that. Maybe the legendary staff from old Naxxramas would do that for Karazhan (I don't know where the portal it makes goes, inside or outside), but good luck finding one to test!

There is also the social solution that I've often used, of finding a friend or paying someone a marginal bit of gold to form a raid. Maybe it's dumb, maybe it's an interesting problem to work around. I don't see the point of an MMO if we're never going to need other people.

@Kring: That's not why I solo it. I solo it because if my friend and I are after the same item, it makes no sense to cut in half the potential number of drops. Sometimes it's an activity I can do when there are few friends online.